An organic EL element is a semiconductor element that converts electric energy into light energy.
In recent years, researches on an organic EL element which is attempted to be applied particularly to a display screen of a mobile phone or a portable display device have been actively conducted. Moreover, improvements have been made to organic materials constituting the organic EL element and the like, and as a result, the drive voltage of the organic EL element has been remarkably lowered and the luminous efficiency has been heightened. From this, an organic EL panel in which an organic EL element highly increased in luminance and highly enhanced in efficiency is adopted as an illumination panel has been put into practical use and has begun to be sold in the market.
In the case of using an organic EL panel as an illumination device, the organic EL panel is required to have high luminance and high luminous efficiency. In order to attain high luminance, it is necessary to make the applied current large and to make the current density per light-emitting area large. However, although the current density per light-emitting area applied to an organic EL panel can be made large to attain high luminance, in contrast thereto, there is a problem that a load is applied to an organic EL element built in the organic EL panel and the life of the organic EL element is liable to be shortened. On that account, Non-Patent Document 1 discloses a lamination type organic EL element including a charge generating layer as an improvement method therefor.
Incidentally, in recent years, with regard to a next-generation illumination panel, the technological development of a method for imparting a panel with high color rendering properties at various color temperatures has been actively conducted.
However, in the current situation of the white light-emitting organic EL panel, sufficiently high color rendering properties have not yet been realized as compared with a conventional fluorescent light. Particularly, in the case where an organic EL panel is made to have a luminance adjusting function and the current density is changed to make the organic EL panel variable in luminance, the color temperature is difficult to be maintained constant due to a change in current density at the time of changing the luminance. As such, in order to realize a white light-emitting organic EL panel having both high stability and high color rendering properties, some challenges still remain to be done.
In this context, with regard to a white light source with three primary colors of red/green/blue, it is advantageous for an increase in luminance and an enhancement in efficiency to heighten the emission intensity of green color with a large luminosity factor. As such, in a conventional panel, it is common for the maximum emission position (hereinafter, also referred to as peak wavelength) of the emission spectrum of green color and the peak wavelength of the emission spectrum of red color to be closely arranged so that the emission spectra of green color and red color are overlapped.
However, in such a structure, the luminance can be enhanced, but the general color rendering index Ra is not more than 80 or so and there has been a limit to the reproducibility of a natural hue of an object to be illuminated. Particularly, in the conventional panel, there has been a problem that the special color rendering index R9 remains 20 to 30 or so and red color fails to be vividly reproduced.
Moreover, when the peak wavelength of a green color phosphorescent light-emitting layer is arranged close to the peak wavelength of a red color phosphorescent light-emitting layer, the emission spectrum of the green color phosphorescent light-emitting layer is made to shift to the red side and reddish green color is emitted from the green color phosphorescent light-emitting layer. That is, when the red shift occurs at the green color phosphorescent light-emitting layer, it is disadvantageous for an enhancement in efficiency and there is also a problem that the phosphorescence luminescent material, which is relatively expensive, is wastefully consumed.
On that account, in Patent Document 1, there has been proposed an organic electroluminescent element having two maximum emission wavelengths in the emission spectrum of blue color ranging from 440 to 490 nm, one maximum emission wavelength in the emission spectrum of green color ranging from 500 to 540 nm and one maximum emission wavelength in the emission spectrum of red color ranging from 600 to 640 nm, wherein the maximum emission wavelength of green color and the maximum emission wavelength of blue color are arranged so as to have a certain distance or more therebetween and the maximum emission wavelength of green color and the maximum emission wavelength of red color are arranged so as to have a predetermined distance or more therebetween. According to the element of the Patent Document 1, without making the red shift occur, the general color rendering index Ra higher than that of the conventional panel is attained.